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Multipurpose simple interface system


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My Pigeons all wiggled, my rose dropped its' petals and my cue lights work.....


Time for another project! (Can you tell I'm bored?)


One of the most common tasks I get asked to do as resident electronics/electrical engineer at the theatre (amateur) is wire up various switches/lamps/detectors built within the set. A few recent examples;

Switch on a light above a head-sized door every time it's opened.

A pressure pad/safety switch arrangement to show a performer is clear of the scissor lift. ("stand here and only press this once you're off")

A detector to show when a miniature train has passed by, then play a sound effect (Halloween at a miniature railway!)

Trigger an air blast solenoid when this button is pressed.


The list of general, basic interfacing jobs goes on.... Sometimes I have to do several outputs from one input, or vice-versa.


Each time I end up building an off the shelf solution that normally involves large amounts of electrical tape and heat shrink to package it up.


I'm now looking to do something a little more elaborate;


I foresee a number of devices, call them building blocks, that can be connected together to build an interface system for whatever temporary event I'm involved in. These would be connected in a chain by XLR cables. Inspiration taken from the Canford opto switchers. So far I see the following blocks being useful;


Power supply (24v)

Switch module (triggers the system)

Input interface (as above, but with banana plugs/screw terminals for connection of a microswitch/pressure pad/whatever)

Mains relay (probably with IEC in and out - I don't need more than 10A)

Low voltage relay (with the option of local power supply, or using the system supply)

CD player fader start interface

Mouse left click (hacked mouse, USB connected so another mouse can be used to position the cursor over the necessary button)

Keyboard interface (selectable hot keys from multiplay assignable to a relay closing contact)


Cheap smoke machine control (for my el-cheapo IEC control smoke machine)


Then some simple interlocks;

Local interlock (3 x XLR connections – chain in, chain out (to next device) and interlock chain. Only when the interlock chain is switched on (by a switch module or input interface) would the chain out become active. This would be switchable between being an “AND” function – IN chain has to be switched at the same time as Interlock chain, and a chain break - IN chain has no effect on OUT).




Cabling wise, I foresee it using 3 pin XLR cabling, with the following pinout;


1 – ground


2 - +24vDC


3 – switch (ground pull-down)


I know 3 pin XLR’s are frowned upon for anything other than audio, but in this instance it’s a costom solution made just for me. I have tones of 3 pin XLR’s, which is part of the attraction. Some of these boxes could be positioned a long way away from each other (hence 24v, rather than 12) and the ready availability of this cabling is a big bonus.


A future addition might be a DMX interface to trigger the chain by DMX. This would allow a modular system of interfaces to be connected to the DMX card for whatever job I needed them to do – one day I could DMX control an analogue smoke machine, the next it could be a cd player fader start, and the next I could advance a powerpoint slide with a mouse interface. Or all the above at once.




A few other thoughts – the interfaces will all (where relevant) be selectable as normally open or normally closed so a switch signal can turn something on or off.




The aim is to have a standard design and project box for each one, then just make more as they’re needed until I’ve got a tool kit of interfaces for whatever purpose.




The question is, can anybody spot any other useful interfaces I’ve missed? Can anybody see any major flaws in the design? Hopefully I can make each interface for around £20. Of course this will vary slightly depending on the task.


I’ll post photos and drawings as and when I get to a suitable stage. I now just need to pick a suitable project box and standard set of relays for mains and low voltage. PSU’s are sorted – I have a really nice pair of Mascot medical grade 24v units.


Once I have a standard pinout, I can build individual projects and then if needs be, strap them together as part of a larger system.



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Sometimes I have to do several outputs from one input, or vice-versa.


You didn't mention having a module to link multiple inputs together (via logic circuits, obviously). Just wanted to point out as well (probably unnecessarily) that if you have AND logic, you could switch inputs out of the circuit by holding them high.



Input ------------ logic
        / <-switch to allow input to be used or not used
+24v -----

(Not including resistors)


Similarly, you didn't mention several outputs to one input- could be done by passively splitting, but you may want to make a module to do that with including amplification, etc?





E2A: Oops, just reread the OP, and realised that you'd written about that already.

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Before you build this these people have built, or are building, a product called "Sound Stage" which does exactly what you are proposing, but will incorporate an audio player as well. The last time I saw it was in prototype format in a suitcase, but thats about a year ago now and it should be nearly ready.


As they are in Crossgates, Leeds they aren't too far away to go and have a nosey, and they are nice people. Even if it turns out more money than you want to spend it may give you ideas to incorporate into your project.

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Thanks both, 2 quite interesting thoughts there.


Yes I'd been thinking about ways to combine and split signals, and doing some basic logic. My plan so far is to use all relay logic if I can. Just keeps things a bit more simple and I don't think it needs much more, plus PCB's aren't required then. An AND module is an interesting idea. The interlock kind of gives the same functionality - if it's set to AND mode then the upstream chain has to be in a trigger state AND the interlock chain. There's no reason why the interlock chain can't have another Interlock unit strapped to it. Nevertheless I can now foresee a need for an AND module, maybe with 4 inputs, but switchable to be any number less than that (3 or 2).


The great thing about this is the bulk of the cost is in the plastic project box. Everything else is just XLR's relays and switches.


For speed of build, but also to make installations a bit simpler, I'm thinking of doing short flying leads with IEC's on the ends for the mains relay units. Saves cutting the boxes for IEC panel mounts and means all I need is a couple of glands and a 16 or so mm hole. I guess powercon would be an option, but this isn't such a high-end device to need it I don't think, plus they cost a lot more!



I've had a look at the SLRetail website, some interesting stuff and I might get in touch, but if I'm honest I quite fancy the project of doing it myself! Perhaps if their product does come to market, an interface device could be used. With the availability of mp3 player pcb's, it might be a function mine could have in the future.


Other interfaces that have come to mind include;

Call light flasher (on a cue light system) - signal the comms system if something happens. Possibly of little use, after all, nobody would know if somebody had just caught their switch or not, but there might be mileage in it somewhere.....


This one's still in its' infancy, and I need to do a bit more thinking (and consulting) on how good an idea it is or not, but maybe a pyro interlock? This WOULD NOT fire the pyro circuit, but it would prevent the circuit from being fired should a button not be pressed (dead man's handle). Lots to think about in terms of safety and making sure a fault can't fire the pyro, but again I think there's mileage in it.


The beauty is, as I need an interface, I build it. I've already put together a list of standard parts, so I can make them all to a common design. The engraving system at work is also going to prove helpful as I can put a nice professional looking engraved legend on each one.


I did wonder if it was possible to build a dual channel system whilst still providing +24v and 0v at all times on the XLR line to power interfaces that need permanent DC (latching relays, etc). It's possible if I use a split rail PSU and both + and - 24v with diodes in each interface. But..... I'd prefer to use a ground pull down, and the effort taken to put in 2 XLR's Vs one is tiny. Perhaps one day a multiplexed control (hell, why not use DMX) to control multiple remote systems from one cable. I think that's a long way in the future though!

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I'd consider using a terminal post system, as you would in school science labs, but build breakout boxes / cables that let you patch down XLR cables in any configuration appropriate to give you the cable runs - that way you can build any circuit that you could locally - relays/switches in series to create 'AND' circuits (or multiple break points), two switches (and a common) down one XLR3 (as long as the PSU was then local to the output devices, or you patched again down two different XLR cables), patch locally without using XLR cables, interface into bare ends with the terminal posts, etc etc.
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I do some of this quite frequently, using just a MididTron interface and PCStage. Its not as flexible as could be required (ie no logic) but its certainly good enough for three of the four projects you mention. Stuff like open the door and you hear the wind whistling outide. On stage lightswitches that appear to work.


The exception is the scissor lift; thats getting into scary territory, with rules and regulations. I'd suggest you borrow John Huntingdon's book from your local library, you'll find lots of information and not a little inspiration in there.


The critical thing for doing this level of integration is making sure it works when required and doesn't work when it shouldn't.

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Indeed, and the nature of my day job means I'm all too well aware that when things are meant not to work, they most definately don't! It's one reason why relay logic is looking like the way forward. If it's good enough for instrument landing systems and man aloft inhibits on 1MW radar systems, it's good enough for my little grott boxes! That Mididtron kit looks absolutely fantastic, especially the wireless version. The wired version isn't exactly expensive either. I'll bear them in mind....


In terms of the lift - I don't foresee the thing actually controlling anything heavy like that - I ain't going there! Indication lamps to display conditions to the operator (who would also be able to see what was going on) would be the way forward here as an additional level of safety. Ensuring such a system "failed safe" - that is, couldn't give a safe indication if it wasn't, would be the order of the day.


As with all these things, keeping it simple is often the safest way forward. All the interlocks and control systems I've designed at work have been painfully simple because they're at least predictable.


I did wonder about the availability of te config switches that set things like NO or NC on the relays. These will be on the boxes themselves. They'll each also have a local trigger switch for test purposes (triggering that relay or function without triggering the whole chain). I wondered if the availability of these switches would allow people to fiddle/make a system unsafe. My thoughts however are that you'd assemble this thing and then tuck all of the boxes out of the way somewhere, so only the user controls would be available, be they microswitches wired back for a specific install, or general purpose switch boxes with nice big arcade buttons on them. You could maybe even assmeble the system in some sort of case/box that allowed cables to pass in and out, then when it was working and set up, seal the box so it can't be messed.

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I've also used the Highly Liquid controllers before for triggered MIDI inputs into QLab - the other thing I've used is butchered Korg Nano units - they're about £30/piece for direct bus-powered USB inputs, and have the MIDI usb controller in there as well, whilst the Highly Liquid unit naturally requires an external MIDI interface - you could use the internals out of a Korg Nanopad or similar to give you a USB midi device interfaced into your system to trigger software.
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Good calls on the midi ideas. Just at the minute that's maybe a little further down the line, but I've now properly designed the first 3 boxes (on the back of a scruffy piece of A4, I can do neat copies on pc afterwards, but my head doesn't work that way when I'm designing circuits - pencil and paper all the way!).


So a brief description of the 3 units;


Mains relay - Power on LED, Unit triggered LED, Local trigger switch (triggers just that interface, not the line), NC/NO selector switch, local mains fuse.


Low voltage relay - Power on LED, Unit triggered LED, Local trigger switch, then it all gets a bit different - I present all the relay contacts as well as the local supply and ground on banana plugs - then when building a system you just plug into the ones that suit most - meaning you can tap off the chain supply (fused) or use your own local battery/whatever on suitable flying leads with banana plugs on the end. (When I say banana sockets - I mean the ones that'll also screw onto a wire, like the old school lab packs).


Input module - This one I'm really pleased with, but it might need a brief description. Power on LED, Unit triggered LED, Trigger switch input (banana sockets) and local switch on box, Line NC/NO selector switch, Trigger NC/NO selector switch, Latch mode selector switch, Latch reset input (banana sockets) and local switch, Latch reset NC/NO selector switch.


To describe the functions of the above one, in non-latch mode the trigger switch works a relay. The trigger NC/NO selector does exactly that - think microswitch on a door Vs switch pressed by a performer. The Line NC/NO selector switch determines whether the line is normally triggered or not (remember though that this doesn't mean all interface outputs have to be on - there's local control of this on all the relay boxes. I envisage that line not triggered is the normal state for the duration of whatever the system is doing). In non-latch mode the reset button (used later) actually functions as an inhibit switch to the trigger (again, NC/NO selectable) so it's a simple interlock device as well.

In latch mode, all the NC/NO functions above still exist, but a single momentary switching of the trigger input latches the line on (or off, depending on the NC/NO switch!!!) and then the reset switch switches it back the other way.

Another function I'm still wondering about is a toggle, so the trigger toggles the line on or off. This circuit is slightly more involved than relay logic though. Would be a useful function to have.


I know the input device looks complicated, but remember it's designed to allow for all eventualities and be set up once by somebody that know how it all works, then be hidden away where it can't be fiddled with.

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On the input you need to be able to detect a change of state. Its handy making the on-stage ligfhgtswitches work, but sometimes the switch starts out at the wrong position, and the effect still needs to work properly...
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